Device and method for separating plasma from a blood product

ABSTRACT

Plasma is separated from a blood product such as SAG-M or anticoagulated whole blood by flowing the blood product across a membrane. The membrane has a voids volume of greater than 50%, a pore size of less then 0.65 μm and a plasma flow rate of greater than 0.04 ml/min.cm 2  with a specified value of blood product and with a specified transmembrane pressure. The surface of the membrane contacted by the blood product has a smoothness of less than 0.5 μm.

This application is a 371 of PCT/GB94/01256 filed Jun. 10, 1994.

The invention relates to a device and method for separating blood cellsfrom plasma or from another fluid in which cells are suspended.

An adult human contains about 5 liters of blood, of which red bloodcells, also referred to as erythrocytes, account for about 45% of thevolume, white cells about 1% and the balance being liquid blood plasmain which the cells are suspended. Blood also contains large numbers ofplatelets suspended in the plasma (their proportional volume is small).In view of the substantial therapeutic and monetary value of bloodcomponents, such as red blood cells, platelets and plasma, a variety oftechniques have been developed to separate blood into its componentfractions or to separate combinations of such components while ensuringmaximum purity and recovery of each of the components.

Throughout this specification the term blood product will be used torefer to anticoagulated whole blood or suspensions of red blood cells(with or without other blood cell types) in a suitable fluid such asplasma or SAG-M (whose composition is detailed below).

In general, such separations have been achieved by centrifugationtechniques. This requires, however, significant handling of the bloodproduct, which can increase the risk of disease transmission. Inaddition, centrifuging also takes significant time.

An alternative to centrifugation for the separation of red blood cells,white blood cells and platelets from plasma or another fluid in whichthe cells are suspended is filtration. This can be achieved by flowing ablood product across a surface of a membrane whereupon plasma or anothercell suspending fluid passes through the membrane under a pressuregradient generated across the membrane. This process is hereinafterreferred to as cross-flow filtration. It has been found, however, thatthe filtration efficiency of such a membrane drops because the membranepores become blocked with blood cells and cell fragment debris duringfiltration. Blocking cells and debris may not be effectively removed bycross-flow of the blood product across the surface of the membrane. As aresult, the membranes previously used for such filtration have blockedbefore the haemocrit (the percentage of red blood cells in thesuspending fluid) reaches a desired figure, for example 70% by volume.

In EP-A-464707, this problem is sought to be overcome by a shearingforce induced at the membrane surface by forming the membrane as acylinder and rotating it within a non-rotating outer cylinder whosewalls are in close proximity to the rotating membrane. This processgenerates so-called "Taylor Vortices" which are intended to clear themembrane surface of clogging cells and debris and to provide increasedtrans-membrane pressure to increase plasma flow across the membrane.

A similar proposal for overcoming this problem is made by Beaudoin andJaffrin in the article "Plasma Filtration in Couette Flow MembraneDevices" in the Journal "Artificial Organs" Volume 13 No. 1 1989 pages43-51.

EP-A-0111423 endeavours to induce vortices in the flow using dimplesformed on the surface of the membrane.

These approaches suffer from certain technical limitations. The proposalof EP-A-0111423 requires very precise alignment of the "dimples".Failure to achieve this will degrade the effectiveness of the device.The approaches of Beaudoin and Jaffrin and EP-A2-464707 require accuratealignment of the rotating and non-rotating cylinders and additionallyrequire an electric motor to rotate the rotating cylinder whichincreases the expense and complexity of the device.

In all these proposals for plasma separation by filtration, it isessential that there is no or no significant lysis of erythrocytes inthe blood product, since this releases haemoglobin. Since this is aprotein, it can pass freely across the membrane with the othernon-cellular components of the blood. Free haemoglobin (i.e. haemoglobinnot contained within the erythrocyte cell membrane) is potentiallyundesirable, especially where the plasma is required for therapeuticpurposes. Additionally, erythrocyte lysis produces cell fragments whichcan block the membrane pores.

Such lysis can be avoided or mitigated by the use of membranes with verysmall pore sizes (≦0.1 μm) i.e. considerably smaller than is requiredsimply to prevent erythrocytes crossing the membrane. However, suchmembranes currently in use do not allow flow rates of plasma across themembrane that are sufficiently high to make separation of plasma byfiltration a viable option--particularly when compared to centrifuging.

According to a first aspect of the invention, there is provided a devicefor treating a blood product comprising red blood cells suspended in afluid whereby to separate the fluid from the cells, the device having aninlet for the blood product, an outlet for the separated fluid with amembrane disposed between the blood product inlet and the fluid outlet,the membrane having a voids volume of at least 50% and a surface havinga smoothness (as herein defined) of less than 0.5 μm.

According to a second aspect of the invention, there is provided adevice for treating a blood product comprising red blood cells suspendedin a fluid whereby to separate the fluid from the cells, the devicecomprising a housing having an inlet for the blood product, an outletfor fluid depleted cells, an outlet for fluid and a membrane disposed inthe housing separating the fluid outlet from the blood product inlet andthe fluid depleted cell outlet so that blood product flows across asurface of said membrane becoming fluid depleted, the membrane having apore size of less than 0.65 μm, and wherein the volume of fluidseparated over 15 minutes is greater than 0.6 ml per unit area in cm² ofthe said surface when the device is used to treat a volume correspondingto 3.75 ml per unit area in cm² of the said surface of red blood cellssuspended in SAG-M solution at a haematocrit of 45%, and when thetransmembrane pressure difference is 35 mbar, the said membrane surfacehaving a smoothness (as herein defined) of less than 0.5 μm.

According to a third aspect of the invention, there is provided a methodof treating a blood product comprising red blood cells suspended in afluid whereby to separate the fluid from the cells comprising filteringthe blood product with a membrane, the membrane having a voids volume ofat least 50% and a surface with a smoothness (as herein defined) of lessthan 0.5 μm.

According to a fourth aspect of the invention, there is provided amethod of treating a blood product comprising red blood cells suspendedin a fluid whereby to separate the fluid from the cells comprisingflowing the blood product across a surface of a membrane, the membranehaving a pore size of less than 0.65 μm, and a smoothness (as hereindefined) of less than 0.5 μm and wherein the volume of fluid separatedover 15 minutes is greater than 0.6 ml per unit area in cm² of the saidsurface when a volume of blood product corresponding to 3.75 ml per unitarea in cm² of the said surface is being treated, and when thetransmembrane pressure difference is 35 mbar.

The following is a more detailed description of some embodiments of theinvention, by way of example, reference being made to the accompanyingdrawings in which:

FIG. 1 is a cross-section of a device for separating plasma from blood,

FIG. 2 shows two traces from a Mitutoyo, Surftest 401 surftest machine,the upper trace being the surface trace from a smoother membrane and thelower trace the surface trace of a less smooth membrane,

FIG. 3 is a graph plotting against time, firstly plasma flow rate (lefthand y-axis) and secondly haematocrit (right hand y-axis) for an exampleof a blood product filtered through a membrane,

FIG. 4 is a graph plotting the amount of haemoglobin in the plasmaagainst time for the Example of FIG. 3.

Referring to FIG. 1, the device comprises a plasma outlet member 10, amanifold 11 and an insert 12.

The plasma outlet member 10 has a generally annual base 13 with a flatupper surface 14. A plasma outlet 15 leads from the centre of thissurface 14 and connects with a passage 16 which terminates at an outersurface of the plasma outlet member 10.

The flat upper surface 14 is surrounded by an annular wall 17 which inturn leads to an annular L-shaped rebate 18 extending around the surface14.

The manifold 11 has an annular body 19 which is co-axial with the axisof the surface 14 and the wall 17 and rebate 18 and is received in theL-shaped rebate 18 and includes an outer axially extending annularsurface 20 carrying an annular seal 21 which seals against the axialsurface of the L-shaped rebate 18.

The manifold 11 also includes a depending annular flange 22 having anannular outer wall 23 in contact with the annular wall 17 of the plasmaoutlet member 10.

The manifold 11 includes a blood inlet 24 and a blood outlet 25. Theblood inlet 24 includes a passage 26 extending through the body 19 at anangle to the axis of the manifold 11 and leading to a passage 27 whichextends through the flange 22 in a direction parallel to the axis of themanifold and which terminates at an outlet 28 adjacent the flat uppersurface 14 of the plasma outlet member.

The blood outlet member 25 is formed with an inlet 29 adjacent the flatupper surface 14, a first passage 30 extending through the flange 22 anda second passage 31 extending through the body 19 at an angle relativeto the axis of the manifold 11.

The manifold 11 carries the insert 12 which is generally cylindrical inshape and co-axial with the manifold axis with an outer cylindrical wall32 carrying a seal 33 which contacts an inner annular wall of themanifold 11 to form a seal therebetween. The insert 12 also has acircular head 35 provided with ports 36a,36b. One port 36a extendsthrough the head 35 and is coaxial with the common axis of the manifold11. This central port 36a is also in alignment with the axis of theplasma outlet 15. The other ports, 36b, are arranged around the junctionbetween the head 35 and the cylindrical wall 34 of the insert 12 and areat an angle to the common axis 37.

A lock nut 38 is in threaded engagement with the outer wall 34 of theinsert 12 to allow the gap between the head 35 and the flat uppersurface 14 to be adjusted.

An annular seal 39 extends around the outer periphery of the flat uppersurface 14 and engages an outer edge of the flange 22--but is separatedfrom an inner edge of the flange 22 as seen in the drawing to provide aflow path therebetween.

In use, the device is disassembled by removing the manifold 11 and theinsert 12 from the plasma outlet member 10. An annular disc of membranematerial is then placed on the flat inner surface 14 with its axiscoaxial with the axis of the plasma outlet 15 such that the membranesurface across which the blood product is to flow is positioned upwards.The diameter of the membrane 40 is such that its outer peripherycontacts the inner periphery of the annular seal 39.

The manifold 11 and the insert 12 are then re-engaged with the plasmaoutlet member 10 and the position of the head 35 relative to the flatinner surface 14 is adjusted to a desired spacing by use of the lock nut38.

After prewetting of the membrane with saline a blood product is thenpassed or circulated across the upper surface of the membrane 40;passing from the blood inlet 29 to the blood outlet 28. The pressuregradient across the membrane 40 maintains a flow of plasma or anotherfluid in which the cells are suspended through the membrane which thenleaves via the plasma outlet 15 and the passage 16.

The blood flow can be provided by a peristaltic pump or a syringe pumpor may also be provided by the use of air or mechanical pressure appliedto a container of blood product.

A known device for testing ultrafiltration membranes is sold by RhonePoulenc under the trade mark RAYFLOW PLEIADE as a cross flow filtrationjig. A housing contains two membranes carried on respective oppositefaces of a support. The product to be filtered is flowed across thesurfaces of the membranes not contacting the support from respectiveinlets to respective outlets, and the filtrate is extracted from thesupport.

It has heretofor been thought that the only significant parameter for amembrane suitable for separating plasma from a blood product is the poresize which, as discussed above, has been required to be small enough toprevent erythrocytes passing through the membrane and to preventhaemolysis. However, as also discussed above, it has been found inprevious membranes used for plasma separation and meeting thisrequirement, that the plasma flow rate is not sufficiently high to makesuch separation commercially viable.

The separation of plasma from a blood product at useful plasma fluxrates and with the avoidance of lysis requires, it is herein postulated,a combination of the smoothness of the membrane surface contacted by theblood product and the structure of the membrane--the structuredetermining the rate at which a plasma passes through the membrane.

The pore size determines the sizes of particles that will be allowed topass through the membrane. However, the flow rate of plasma through amembrane is also controlled by the structure of the membrane includinginternal structure of the pores. Accordingly, it has been found thatmembranes of different constructions but having the same pore size havediffering flow rates.

Flow rate of plasma is important in crossflow plasma separation devicesbecause it is naturally desirable to separate the plasma as rapidly aspossible. In addition, a crossflow device is only likely to becommercially acceptable if the processing time for a blood product is nolonger than that of a centrifugal separator (for example a haematocritof 70% in 10-15 minutes).

One accepted measure of the "openness" of the internal structure of amembrane is the "voids volume" (porosity) of the membrane. This is thepercentage of the membrane volume which is not occupied by the polymersubstrate. The voids volume of membranes can vary from as little as 5%to in excess of 80%.

It has been found, however, that membranes which have high voids volumes(above 50%) and so have acceptable flow rates also have such a tendencyto cause lysis when filtering plasma from a blood product, that thequantity of haemoglobin in the plasma is so great as to be unacceptable.It is believed that this is due to the fact that membranes with higherflow rates also have a more "open" surface structure which can providesites where erythrocytes can be subject to lysis and the release ofhaemoglobin, and where cellular debris can collect and block the pores.

It is now believed that this problem can be mitigated by increasing thesmoothness of the surface of the membrane in contact with the bloodproduct. It is believed that this reduces the incidence of lysis and thecollection of cell debris by removing sites on the membrane surfaceresponsible for such effects.

The device described above with reference to FIG. 1 and the RAYFLOWPLEIADE device referred to above were used to perform comparative testson various different materials for the membrane. In broad terms, thetests were divided into two groups. In the first tests, using theRAYFLOW PLEIADE device, a membrane in accordance with the invention wastested against a commercially available membrane for filtration of bloodproducts and a control membrane. The tests are for demonstrating that,although in both the membrane in accordance with the invention and thecommercially available membrane, lysis of the blood product was kept atan acceptably low level, the time-averaged flow rate of plasma per unitarea of the membrane was higher in the membrane in accordance with theinvention than in the commercially available membrane.

In the second tests, using the device of FIG. 1, a membrane inaccordance with the invention was tested against membranes havingsimilar pore sizes and flow rates but having a less smooth surface incontact with the blood product. The purpose of the test was to show thatthe membranes with less smooth surfaces produce unacceptable lysis.

In these tests, various measurements were used, as follows.

PLASMA FLUX RATE

The average rate of flux of plasma or of another suspending fluid suchas SAG-M (PFR) across a membrane is referred to as the plasma flux rate(PFR) and was measured as the volume in milliliters of plasma or fluid(Vp) produced by the device over a set time period (t) in minutes perunit area (A) in cm² of the membrane being tested. Thus ##EQU1##

The rate is measured using a set volume (V_(b)) of blood product becauseas plasma is extracted from the blood product the flow characteristicschange so that a time averaged plasma flow rate will be greater from agreater volume of blood product than from a lesser volume of bloodproduct. Additionally it is necessary to define the surface area of themembrane across which the blood product passes as the extraction ofplasma will occur faster with a larger surface area.

HAEMOLYSIS

The degree of haemolysis (H) is measured as the content of haemoglobinin the plasma or other cell suspending fluid produced by the device.This can be measured subjectively by a visual inspection of the plasma(which will become increasingly red as the volume of haemoglobin in theplasma increases) or can be measured by known methods in milligrammes ofhaemoglobin per milliliter of plasma or other fluid. In the former case,the degree of haemolysis) can be expressed either as "-" (meaning noobserved haemolysis) to "+++++" (meaning severe haemolysis), with valuesin between being expressed by from "+" to "++++".

BLOOD PRODUCT

The tests were conducted using a blood product which was eitheranticoagulated whole blood or blood cells suspended in SAG-M. SAG-M isan aqueous additive solution containing sodium chloride 140 mmol/l,adenine 1.5 mmol/l, glucose 50 mmol/l and mannitol 30 mmol/l. Bloodcells (mostly red blood cells) were separated from anticoagulated wholeblood by conventional centrifugation techniques and suspended in SAG-Mto give a haematocrit value similar to that of whole blood(approximately 45%).

BLOOD VELOCITY

The tests were conducted with the whole blood or the cell suspension inSAG-M pumped through the device by a pump which produces a known flowrate (F) of whole blood or SAG-M through the device. F was measured inmeters/second. The blood velocity is a significant parameter becauseincreasing the blood velocity tends to prevent debris from attaching tothe membrane surface and so increases PFR.

TRANSMEMBRANE PRESSURE

The transmembrane pressure (P) was maintained at a known level measuredin mbar g, which is determined by the blood flow rate through the deviceand the restrictions on the outlet for the blood product and the plasmaoutlet.

SMOOTHNESS

The smoothness of a membrane surface was measured using a MitutoyoSurftest 401 tally surf machine sold by Mitutoyo (UK) Company Limited.In such a machine a stylus is drawn across the surface of the membranebeing measured and variations are measured in the position of the stylusin directions normal to the surface. The smoothness of the surface isquantified by an average deviation of the stylus position from a meanposition in μm. When used in this specification, a measurement"smoothness" means a measurement made in this way.

The first group of tests using the RAYFLOW PLEIADE device will now bedescribed. They are designed to demonstrate that a device in accordancewith the invention has a superior plasma flux rate (PFR) as comparedwith another membrane previously used for plasma filtration, eventhough, in both cases, the degree of lysis was acceptable. A controlmembrane was also tested.

EXAMPLE 1 Invention

The membrane material used was a 0.2 μm pore size nylon 66 membrane soldby Pall Corporation under the trade mark ULTIPOR N₆₆ and cast on Mylarfilm as described in U.S. Pat. No. 4,340,479. After casting two piecesof membrane were peeled off the Mylar and heat bonded together in faceto face contact with the Mylar cast surface outwards.

The blood product was repeatedly passed through the RAYFLOW PLEIADEdevice over the course of the experiment--the suspension of cellsemerging from the device being recirculated through the pump and back tothe device. The other test conditions were as follows:

V_(b) =450 ml

t=15 minutes

A=120 cm²

Blood product=blood cells suspended in SAG-M

Blood velocity (F)=0.33 m/sec

Transmembrane pressure (P)=35 mbar g

The resultant PFR is shown in Table 1 below.

EXAMPLE 2 Control Membrane

The membrane material used was a 0.2 μm pore size nylon 66 membrane soldby Pall Corporation under the trade mark ULTIPOR N₆₆ and not cast on aMylar film. The remaining test conditions were as in Example 1.

The resultant PFR and haemolysis are shown in Table 1 below.

EXAMPLE 3 Prior Art--Known Plasma Separation Membrane

The membrane material used was a commercially available 0.2 μm pore sizepolycarbonate membrane used for plasma separation. The remaining testconditions were as in Example 1.

The resultant PFR is shown in Table 1 below.

                  TABLE 1    ______________________________________    EXAMPLE NO.    PFR    VOIDS VOLUME    ______________________________________    1              0.119  >70%    2              0.079  >70%    3              0.036  <50%    ______________________________________

It will be seen that the membrane of Example 1 had a significantlygreater PFR than the membranes of Examples 2 and 3. As noted above, themembrane of Example 1 was a membrane of nylon 66 cast on Mylar and soldby Pall Corporation under the trade mark ULTIPOR. This nylon 66 membraneis characterized by having a comparatively high internal voids volume(70% or greater) and a comparatively high density of surface pores withno substrate. The membrane of Example 3 has a lower voids volume and/orhas a comparatively low density of surface pores or has a substrate.These factors tend to reduce significantly the PFR. The membranesproduced acceptable levels of lysis with the exception of Example 2which was used as a control.

It will also be appreciated that the PFR achieved by Example 1 requiresonly the transmembrane pressure created by blood flow between astationary membrane and a stationary adjacent surface. The PFR does notrely on the use of increased blood velocities arising from relativemotion between the membrane and the adjacent surface, as proposed, forexample, in EP-A-464707.

The second group of tests using the device of FIG. 1 will now bedescribed. As mentioned above, they are designed to show that, without acomparatively smooth surface in contact with the blood product, lysis oferythrocytes occurs to an unacceptable extent, even though the flowratethrough those membranes is acceptably high.

EXAMPLE 4

The membrane and blood product flow were as described in Example 1. Theremaining test conditions were as follows:

Blood product: Anticoagulated Whole blood

t=15 minutes

V_(b) =150 ml

A=49 cm²

Blood Velocity (F)=0.9 m/sec

Transmembrane Pressure (P)=35 mbar g

In addition the smoothness of the membrane was measured as describedabove. The haemolysis level and smoothness are as shown in Table 2.

EXAMPLE 5

The membrane was a similar membrane to that of Example 4 but having apore size of 0.45 μm. The remaining conditions were as in Example 4. Thehaemolysis level and smoothness were as shown in Table 2.

EXAMPLE 6

The membrane of this example was the same as the membrane of Example 2above. The remaining test conditions were as in Example 4. Thehaemolysis level and smoothness were as shown in Table 2.

EXAMPLE 7

The membrane of this example was the same as the membrane of Example 6except having a pore size of 0.45 μm. The remaining test conditions wereas in Example 4. The haemolysis level and smoothness were as shown inTable 2.

EXAMPLE 8

The membrane of this example was a hydrophilic polyvinyldifluoridemembrane sold by Pall Corporation under the trade mark FLUORODYNE andhaving a pore size of 0.6 μm. The remaining conditions were as inExample 4. The haemolysis level and smoothness were as shown in Table 2.

EXAMPLE 9

The membrane was a nylon 66 membrane sold by Pall Corporation under thetrade mark BIOINERT and having a pore size of 0.8 μm. The remainingconditions were as in Example 4. The haemolysis level and smoothnesswere as shown in table 2.

                  TABLE 2    ______________________________________    EXAMPLE NO.  HAEMOLYSIS  SMOOTHNESS (μm)    ______________________________________    4            -            0.2-0.22    5            +/-          0.2-0.22    6            +++         1.0-1.9    7            ++++        1.0-1.9    8            ++++        0.95-1.45    9            +++++       1.0-1.9    ______________________________________

As seen from Table 2, the degree of haemolysis is much lower with themembranes of Examples 4 and 5 than with the membranes of Examples 6 to9. It is believed that this is due to the fact that the smoother surfaceof these membranes prevents lysis of erythrocytes and consequentlyreduces the release of haemoglobin. It will be recalled that themembrane of Example 6 is the "Control Membrane" of Example 2 and it willbe seen from these Examples that, although such a membrane has a highPFR, its use causes an unacceptable lysis of erythrocytes. It will alsobe seen that increasing the pore size does not reduce the haemolysis butrather increases the haemolysis. It is believed that this is due to thefact that the increase in pore size provides sites where erythrocytescan enter the membrane and be damaged causing lysis and releasinghaemoglobin. It has been found that, in general, membranes with poresizes above 0.65 μm cannot be used for separating plasma from a bloodproduct because, above such a pore size, the pores are sufficientlylarge to permit such entry of erythrocytes. Smaller pore sizes preventsuch entry.

Further tests have been conducted using the device described above withreference to FIG. 1 in order specifically to confirm the effect of thesmoother surface on the PFR and haemolysis.

EXAMPLE 10

In this example, the membrane was a nylon 66 membrane sold by PallCorporation under the trade mark ULTIPOR N₆₆ and cast on Mylar inaccordance with U.S. Pat. No. 4,340,479. The pore size was 0.45 μm. Theother test conditions were as in Example 4. Only one face of themembrane was cast in contact with the Mylar film; the other face was notso cast. From such a membrane, two samples were prepared. The first(SAMPLE 1) was formed by bonding two pieces of the membrane inface-to-face contact with the Mylar cast surfaces in contact. The second(SAMPLE 2) was formed by bonding two pieces of the membrane inface-to-face contact with the Mylar cast surfaces outwards. SAMPLE 1 andSAMPLE 2 were then used for separating plasma from blood product asdescribed above with reference to Example 1 using the same conditions asin Example 1.

FIG. 2 shows the traces from a Mitutoyo Surftest machine for the SAMPLE1 and SAMPLE 2. The upper trace is of SAMPLE 2 and the lower trace is ofSAMPLE 1. It will be seen that SAMPLE 2 is smoother than SAMPLE 1. Themaximum measured deviation for the Mylar cast side was 0.27 μm and forthe non-Mylar cast side 0.47 μm.

Four further samples (SAMPLES 3-6) were also prepared, SAMPLES 3 and 5in the same way as SAMPLE 1 and SAMPLES 4 and 6 in the same way asSAMPLE 2. SAMPLES 1-6 were then tested using the device described abovewith reference to FIG. 1 of the drawings.

Two different samples (No. 1 and No. 2) of anticoagulated whole bloodwere used. Blood sample No. 1 was tested with a single sample of themembrane described above and blood sample No. 2 was tested with twodifferent samples of the membrane described.

The results are as shown in Table 3.

                  TABLE 3    ______________________________________               MEMBRANE            H    BLOOD SAMPLE               SAMPLE              (mg Haemoglobin/    No.        No.         PFR     ml plasma    ______________________________________    1          1           0.112   0.16    1          2           0.147   0.04    2          3           0.101   0.22    2          4           0.150   0.01    2          5           0.106   0.21    2          6           0.157   0.02    ______________________________________

As shown in Table 3, all three membranes with the Mylar cast surfacesoutwards had higher flow rates and lower haemolysis than the membranewith Mylar cast surfaces in contact. This tends to confirm the resultsset out above.

In addition, Example 10 demonstrates the contribution of smoothness toPFR. In this regard, it will be appreciated that PFR is a time averagedrate and that, as plasma is extracted from a blood product so raisingthe haemocrit, the instantaneous plasma flow rate drops (because thereis less plasma to be extracted from the product).

This instantaneous flow rate is also affected by any blockage of thepores of the membrane. Such blockage will occur because of cell debrisand other large particles being trapped in the membrane pores. It willbe seen from Table 3 that, although, of course, SAMPLE 1 and SAMPLE 2have the same water flow rate (i.e. the rate at which clean water willflow through the membrane regardless of the time of flow), the non-Mylarcast surfaces have a lower PFR than the Mylar cast surfaces.

It is believed that this is because decreasing the smoothness mayprovide sites where erythrocytes may lodge and subsequently lyse andwhere cell debris and other particles can lodge, so allowing particlesto accumulate over a period of time and so causing gradual blockage ofthe membrane pores.

This tendency to blockage in membranes with less smooth surfaces can, ifthe smoothness is decreased even further (beyond 0.5 μm), prevent theiruse for extracting plasma from a blood product on a practical scale. Anacceptable level of plasma extraction is such that the haematocrit ofthe blood product is raised to 70% in as short a possible time. Anacceptable time may be measured against the time taken to achievesimilar plasma separation using a centrifruge.

To produce a haematocrit of 70% in a blood product (as typicallyrequired in the processing of blood products) a centrifuge mighttypically take a minimum 30 minutes and so the use of a membrane will bebeneficial if such a haematocrit can be achieved in a similar or lessertime.

The smooth surface membranes (SAMPLES 2, 4 and 6) of Example 10 wereable to achieve a haematocrit of 70% in less than 10 minutes.

EXAMPLE 11

The following further test was conducted to confirm these conclusions.The device described above with reference to FIG. 1 was used with themembrane of Example 1 to filter 150 ml of fresh whole bloodanticoagulated with CPDA under the conditions of blood velocity andtransmembrane pressure of Example 1. The PFR, haematocrit of the bloodand the haemolysis (in mg haemoglobin per 100 ml of plasma) weremeasured at 2 minute intervals for a period of 20 minutes. The test wasrepeated 10 times and for each parameter the mean and ±1 standarddeviations were calculated for each time interval.

The results are shown in FIG. 3 (which plots PFR and haematocrit againsttime) and FIG. 4 (which plots the haemoglobin in the filtered plasmaagainst time).

It will be seen from FIG. 3, that, in the exemplified embodiment of theinvention, the PFR is closely dependent on the haematocrit of the bloodproduct. As the haematocrit rises (i.e. as the concentration of redcells rises) the plasma flux decreases proportionately. This indicatesthat the drop in PFR with time is mainly due to plasma extraction andred cell concentration and not to fouling of the membrane or otherfactors causing a deterioration in membrane performance.

FIG. 4 shows that haemolysis does not begin to increase significantlyuntil 12 minutes have elapsed. With reference to FIG. 3, it will be seenthat after 12 minutes a haematocrit of more than 70% has been reachedand, as discussed above, a haematocrit of 70% is generally regarded asan acceptable level.

Although the haemoloysis increases significantly after 12 minutes, it isbelieved that this is due not to the membrane but to the fact that theshear forces to which the erythrocytes are subjected increase as theviscosity of the blood increases as plasma is removed and thehaematocrit rises. In addition, after 12 minutes, the blood has been incontact with the surfaces of the device for a considerable time whichalso tends to increase lysis as does the action of the pump in pumpingthe red cells at high haematocrits.

It is considered that membrane materials embodying the invention and asdescribed above are less likely to activate platelets.

While the membranes described above have been tested using exemplifiedcross-flow filtration devices, it will be appreciated that they may beused with any suitable device, which may not be a cross-flow device.

We claim:
 1. A cross-flow device for separating a blood productcomprising red cells suspended in a fluid into a separated redcell-depleted fluid and a separated fluid comprising red cells, thedevice including:a manifold having an inlet for said blood product andan outlet for said separated fluid comprising red cells; an outlet forsaid separated red cell-depleted fluid: and a membrane extending acrossthe manifold to one side of said blood product inlet and said outlet forsaid separated fluid comprising red cells; the membrane having a firstsurface and a second surface, the first surface facing the manifold andcontacted by said blood product flowing between the blood product inletand the outlet for said separated fluid comprising red cells; themembrane having a voids volume of at least about 50% and the firstsurface having a smoothness of less than about 0.5 μm as measured by aMitutoyo Surftest 401 tally surf machine as the average deviation indirections normal to said first surface of the position of a stylus ofsaid machine from a mean position of the stylus as the stylus is drawnacross said first surface; said outlet for said separated redcell-depleted fluid downstream of said second membrane surface.
 2. Thedevice according to claim 1 wherein the voids volume of the membrane isat least about 70%.
 3. The device according to claim 1 wherein themembrane has a pore size of less than about 0.65 μm.
 4. The deviceaccording to claim 3 wherein the pore size of the membrane is less thanabout 0.5 μm.
 5. The device according to claim 1, wherein the volume ofsaid separated red cell-depleted fluid separated over 15 minutes isgreater than about 0.6 ml per unit area in cm² of the first surface whenthe device is used to treat a volume corresponding to 3.75 ml per unitarea in cm² of the first surface of said blood product, when said bloodproduct comprises red cells suspended in an aqueous solution includingabout 140 mmol/l sodium chloride, about 1.5 mmol/l adenine, about 50mmol/l glucose and about 30 mmol/l mannitol at a hematocrit of about45%, and when the transmembrane pressure difference is about 35 mbar. 6.The device according to claim 5 wherein the volume of said separated redcell-depleted fluid separated per unit area in cm² of said first surfacein 15 minutes is greater than about 1.2 ml when the device is used totreat a volume corresponding to 3.75 ml per unit area in cm² of thefirst surface of said blood product, when said blood product comprisesred cells suspended in an aqueous solution of 140 mmol/l sodiumchloride, 1.5 mmol/l adenine, 50 mmol/l glucose and 30 mmol/l mannitolat a hematocrit of 45%, and when the transmembrane pressure differenceis 35 mbar.
 7. The device of claim 6 wherein the volume of saidseparated red cell-depleted fluid per unit area in cm² of the firstsurface in 15 minutes is greater than 1.5 ml.
 8. The device of claim 7wherein the volume of said separated red cell-depleted fluid per unitarea in cm² of the first surface in 15 minutes is greater than 2.25 ml.9. The device according to claim 1 wherein the smoothness of the firstmembrane surface is less than about 0.3 μm.
 10. The device according toclaim 1 wherein the membrane comprises a skinless alcohol-insolublehydrophilic polyamide resin.
 11. The device according to claim 10wherein the polyamide comprises nylon 66, said first membrane surfacehaving been cast on a Mylar substrate.
 12. The device according to claim11 wherein the membrane comprises two sheets of 0.45 μm pore size nylon66 membrane, each sheet having been cast on said Mylar substrate andremoved from said Mylar substrate, the two sheets being bonded togetherin face-to-face contact with the Mylar cast sides outward to form saidfirst and second membrane surfaces.
 13. The device according to claim 1comprising a fluid outlet member having a flat upper surface on whichsaid second surface of the membrane rests, the outlet for said separatedred cell-depleted fluid leading through said upper surface; and meansdefining a passage for the blood product from said blood product inletto the outlet for said separated fluid comprising red cells across thefirst surface of the membrane.
 14. The device according to claim 13wherein a wall surrounds said surface of the fluid outlet member, themanifold comprising an annular flange which fits within said wall. 15.The device according to claim 14 wherein the passage defining meanscomprises an insert having a surface spaced from the first surface ofthe membrane, the insert being carried by the manifold.
 16. The deviceaccording to claim 15 wherein said passage defining means is capable ofproviding for adjustable spacing.
 17. The device according to claim 15wherein the will is annular, the manifold is annular and the insertsurface is circular.
 18. The device according to claim 17 wherein thefluid outlet member includes an annular rebate, the manifold includingan annular body received in said rebate with said annular flangedepending therefrom.
 19. The device according to claim 18 wherein themanifold has an annular central bore, the insert being received in saidbore.
 20. The device according to claim 19 wherein a fire seal isprovided between the rebate and the manifold and a second seal isprovided between the bore and the insert.
 21. A cross-flow device forseparating a blood product comprising red cells suspended in a fluidinto a separated red cell-depleted fluid and a separated fluidcomprising red cells, the device comprising:a housing having an inletfor the blood product, an outlet for the separated fluid comprising redcells and an outlet for the separated red cell-depleted fluid; and asubstantially planar membrane disposed in the housing; the membranehaving a first surface and a second surface, the first surface facingthe housing and contacted by the blood product flowing between the bloodproduct inlet and the outlet for the separated fluid comprising redcells; the outlet for the separated red cell-depleted fluid downstreamof said second membrane surface; the membrane having a pore size of lessthan about 0.65 μm; and wherein the volume of said separated redcell-depleted fluid separated over 15 minutes is greater than 0.6 ml perunit area in cm² of the first surface when the device is used to treat avolume corresponding to 3.75 ml per unit area in cm² of the firstsurface of said blood product, when said blood product comprises redcells suspended in an aqueous solution of 140 mmol/l sodium chloride,1.5 mmol/l adenine, 50 mmol/l glucose and 30 mmol/l mannitol at ahematocrit of 45%, and when the transmembrane pressure difference is 35mbar; the first membrane surface having a smoothness of less than about0.5 μm as measured by a Mitutoyo Surftest 401 tally surf machine as theaverage deviation in directions normal to said first surface of theposition of a stylus of said machine from a mean position of the stylusas the stylus is drawn across said first surface.
 22. The deviceaccording to claim 21 wherein the volume of said separated redcell-depleted fluid separated per unit area in cm² of said first surfacein 15 minutes is greater than 1.2 ml when the device is used to treat avolume corresponding to 3.75 ml per unit area in cm² of the firstsurface of said blood product, when said blood product comprises redcells suspended in an aqueous solution of 140 mmol/l sodium chloride,1.5 mmol/l adenine, 50 mmol/l glucose and 30 mmol/l mannitol at ahematocrit of 45%, and when the transmembrane pressure difference is 35mbar.
 23. The device according to claim 21 wherein the pore size of themembrane is less than about 0.5 μm.
 24. The device according to claim 21wherein the smoothness of the first membrane surface is less than about0.3 μm.
 25. The device according to claim 21 wherein the membranecomprises a skinless alcohol-insoluble hydrophilic polyamide resin. 26.The device according to claim 25 wherein the polyamide comprises nylon66, said first membrane surface having been cast on a Mylar substrate.27. The device according to claim 26 wherein the membrane comprises twosheets of 0.45 μm pore size nylon 66 membrane, each sheet having beencast on a Mylar substrate and then removed from said Mylar substrate,the two sheets being bonded together in face-to-face contact with theMylar cast sides outward to form said first and second surfaces.
 28. Thedevice according to claim 21 comprising a fluid outlet member having aflat upper surface on which the second surface of the membrane rests,the outlet for the red cell-depleted fluid leading through said uppersurface, a manifold providing said inlet for the blood product and saidoutlet for the separated fluid comprising red cells, and means defininga passage for the blood product from said blood product inlet to theoutlet for the separated fluid comprising red cells across the firstmembrane surface.
 29. The device according to claim 28 wherein a wallsurrounds said surface of the fluid outlet member, the manifoldcomprising an annular flange which fits within said wall.
 30. The deviceaccording to claim 29 wherein the passage defining means comprises aninsert having a surface spaced from the first surface of the membrane,the insert being carried by the manifold.
 31. The device according toclaim 30 wherein said passage defining means provides for adjustablespacing.
 32. The device according to claim 30 wherein the wall isannular, the manifold is annular and the insert surface is circular. 33.The device according to claim 32 wherein the fluid outlet memberincludes an annular rebate, the manifold including an annular bodyreceived in said rebate with said annular flange depending therefrom.34. The device according to claim 33 wherein the manifold has an annularcentral bore, the insert being received in said bore.
 35. The deviceaccording to claim 34 wherein a first seal is provided between therebate and the manifold and a second seal is provided between the boreand the insert.
 36. A method of separating a blood product comprisingred cells suspended in a fluid into a separated red cell-depleted fluidand a separated fluid comprising red cells using a substantially planarmembrane having first and second surfaces comprising:flowing the bloodproduct across the first surface of the membrane; passing separated redcell-depleted fluid through the first and second surfaces of themembrane; and passing separated fluid comprising red cells across thefirst surface of the membrane; the membrane having a voids volume of atleast about 50% and the first surface having a smoothness of less thanabout 0.5 μm as measured by a Mitutoyo Surftest 401 tally surf machineas the average deviation in directions normal to said first surface ofthe position of a stylus of said machine from a mean position of thestylus as the stylus is drawn across said first surface.
 37. The methodaccording to claim 36 wherein the voids volume of the membrane is atleast about 70%.
 38. The method according to claim 36 wherein themembrane has a pore size of less than about 0.65 μm.
 39. The methodaccording to claim 38 wherein the pore size is less than about 0.5 μm.40. The method according to claim 36 wherein the volume of saidseparated red cell-depleted fluid separated over 15 minutes is greaterthan 0.6 ml per unit area in cm² of the first surface when a volume ofblood product corresponding to 3.75 ml per unit area in cm² of the firstsurface is being treated, and when the transmembrane pressure differenceis 35 mbar.
 41. The method according to claim 40 wherein the volume ofseparated red cell-depleted fluid per unit area in cm² of said firstsurface in 15 minutes is greater than 1.2 ml when a volume of bloodproduct corresponding to 3.75 ml per unit area in cm² of said firstsurface is being treated, and when the transmembrane pressure differenceis 35 mbar.
 42. The method according to claim 41 wherein the volume ofsaid separated red cell-depleted fluid per unit area in cm² of the firstsurface in 15 minutes is greater than 1.5 ml.
 43. The method accordingto claim 42 wherein the volume of said separated red cell-depleted fluidper unit area in cm² of the first surface in 15 minutes is greater than2.25 ml.
 44. The method according to claim 36 wherein the smoothness ofthe first membrane surface is less than about 0.3 μm.
 45. The methodaccording to claim 36 wherein the membrane comprises a skinless alcoholinsoluble hydrophilic polyamide membrane.
 46. The method according toclaim 45 wherein the polyamide comprises nylon 66, said first membranesurface having been cast on a Mylar substrate.
 47. The method accordingto claim 46 wherein the membrane comprises two sheets of 0.45 μm poresize nylon 66 membrane, each sheet having been cast on a Mylar substrateand then removed from said Mylar substrate, the two sheet being bondedtogether in face-to-face contact with the Mylar cast sides outward toform said first and second surfaces.
 48. The method according to claim36 wherein the blood product comprises red cells suspended in an aqueoussolution of about 140 mmol/l sodium chloride, about 1.5 mmol/l adenine,about 50 mmol/l glucose and about 30 mmol/l mannitol.
 49. The methodaccording to claim 48 wherein the hematocrit of the red cells in theblood product is greater than about 30%.
 50. The method according toclaim 36 wherein the blood product comprises anticoagulated fresh wholeblood.
 51. The method according to claim 50 comprising flowing saidblood product across the first membrane surface until a hematocrit ofabout 70% is reached.
 52. A method of separating a blood productcomprising red cells suspended in a fluid into a separated redcell-depleted fluid and a separated fluid comprising red cells using asubstantially planar membrane having first and second surfacescomprising:flowing the blood product across the first surface of themembrane; the membrane having a pore size of less than about 0.65 μm andthe first surface having a smoothness of less than about 0.5 μm asmeasured by a Mitutoyo Surftest 401 tally surf machine as the averagedeviation in directions normal to said first surface of the position ofa stylus of said machine from a mean position of the stylus as thestylus is drawn across said first surface, and wherein the volume offluid separated over 15 minutes is greater than 0.6 ml per unit area incm² of the first surface when a volume of blood product corresponding to3.75 ml per unit area in cm² of the first surface is being treated, andwhen the transmembrane pressure difference is 35 mbar.
 53. The methodaccording to claim 52 wherein the volume of said separated redcell-depleted fluid separated per unit area in cm² of said first surfacein 15 minutes is greater than 1.5 ml when a volume of blood productcorresponding to 3.75 ml per unit area in cm² of the first surface isbeing treated, and when the transmembrane pressure difference is 35mbar.
 54. The method according to claim 52 wherein the pore size of saidmembrane is less than about 0.5 μm.
 55. The method according to claim 52wherein the smoothness of the first membrane surface is less than about0.3 μm.
 56. The method according to claim 52 wherein the membranecomprises a skinless alcohol insoluble hydrophilic polyamide membrane.57. The method according to claim 56 wherein the polyamide comprisesnylon 66, said first membrane surface having been cast on a Mylarsubstrate.
 58. The method according to claim 57 wherein the membranecomprises two sheets of 0.45 μm pore size nylon 66 membrane, each sheethaving been cast on a Mylar substrate and then removed from said Mylarsubstrate, the two sheets being bonded together in face-to-face contactwith the Mylar cast sides outward to form said first and secondsurfaces.
 59. The method according to claim 52 wherein the blood productcomprises red cells suspended in an aqueous solution of about 140 mmol/lsodium chloride, about 1.5 mmol/l adenine, about 50 mmol/l glucose andabout 30 mmol/l mannitol.
 60. The method according to claim 59 whereinthe hematocrit of the red cells in the blood product is greater thanabout 30%.
 61. The method according to claim 52 wherein the bloodproduct comprises anticoagulated fresh whole blood.
 62. The methodaccording to claim 61 comprising flowing said blood product across thefirst membrane surface until a hematocrit of about 70% is reached.